SOTC: Introduction: Are Global Temperatures Rising?

In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change reported that globally averaged combined land and sea surface temperature warmed between 0.65 and 1.06°C from 1880 to 2012 (IPCC 2013). The report also stated, "For the longest period when calculation of regional trends is sufficiently complete (1901 to 2012), almost the entire globe has experienced surface warming." Temperature records supporting IPCC findings have been assembled from thousands of land and ocean observation sites covering a large, representative portion of the Earth's surface, and carefully controlled for possible biases arising from station and instrument changes.

Temperature anomalies by latitude: These graphs track temperature anomalies for northern, low, and southern latitudes. The black dotted line is the annual mean and the red solid line is the five-year mean. The blue bars show uncertainty (95% confidence limit) due to incomplete spatial sampling. Graphs courtesy NASA Goddard Institute for Space Studies Surface Temperature Analysis.

In January 2017, NOAA reported that 2016 was the warmest year in its 137-year series, the third straight year this global annual temperature record has been broken.

Natural cycles of the El Niño-Southern Oscillation contributed to record-breaking warmth from 2014 through 2016. El Niño events generally raise global temperatures, and La Niña events generally lower them. The year 2015 was a strong El Niño year. Although El Niño conditions weakened at the start of 2016, they persisted for several months, with La Niña conditions taking over late in 2016. Despite El Niño conditions dominating in 2015 and 2016, comparing 2015 and 2016 temperatures to those of the last comparably strong El Niño event (1998) shows a marked difference in the temperature departures from average.

Global temperature anomalies, 1880-2016: Global temperatures have exceeded the 20th-century average since 1977. Although strong El Niño conditions occurred in the late 1990s and mid-2010s, temperatures were much higher in 2014 through 2016. Graph courtesy NOAA Climate.gov Beyond the Data.

The difference indicates that the record warmth observed in 2014 through 2016 cannot simply be explained by El Niño conditions in 2015 and 2016. Recent years show a continued trend of increasing temperatures worldwide. In fact, the last year that global temperatures fell below the 20th-century average was 1976.

In its fourth assessment report, the IPCC cited atmospheric concentrations of greenhouse gases as the causative agent in warming temperatures. The panel identified fossil fuel burning and changes in land use as the primary causes of increased carbon dioxide, and agriculture as the primary cause of increased methane and nitrous oxide. Atmospheric carbon dioxide concentrations in 2005 exceeded the natural range for this gas over the past 650,000 years. The IPCC attributed a "greater than 90 percent certainty" to scientists' assertion that higher greenhouse gas concentrations have trapped more thermal radiation and consequently warmed the planet (IPCC 2007).

Oceans dominate the planet's surface, and a comparison of multiple sea surface temperature records—from NOAA, the Hadley Centre, and the Japanese Meteorological Agency—showed that sea surface temperatures rose 0.12°C per decade from 1998 and 2016 (Hausfather et al. 2017).

Temperature record: Global temperature anomalies based on paleoclimate data (green) and the instrumental record (blue) suggest that current temperatures are the warmest in the last millennium, if not longer. Graph courtesy NASA Earth Observatory, adapted from Mann et al., 2008).

Based on land- and marine-based proxy data from collected from across the globe, a 2013 study concluded that the previous decade had exhibited global average temperatures higher than they had been for at least 75 percent of the last 11,300 years (Marcott et al. 2013).

Global temperature anomalies over the past 11,300 years: These temperatures are compared to the 1961-1990 average. The purple line shows the annual anomaly, and the light blue band shows the statistical uncertainty (one standard deviation). The gray line shows temperature from a separate analysis spanning the past 1,500 years. Image courtesy Climate.gov What’s the hottest Earth has been "lately"?, adapted from Marcott et al. 2013.

Is the cryosphere sending signals about climate change?

The cryospheric regions, or regions where water is found in solid form, provide us with direct visual evidence of temperature changes. Unlike other substances found on Earth, ice and snow exist relatively close to their melting point and may frequently change phase from solid to liquid and back again. Consequently, consistent and prolonged warming trends should result in observable changes to Earth's cryosphere. Water changing from solid to liquid and back often results in dramatic visual changes across the landscape as various snow and ice masses shrink or grow.

What are some examples of these snow and ice masses, how do we monitor their conditions, and what do the results show?

In State of the Cryosphere, snow cover, glaciers, permafrost, sea ice, ice shelves, ice sheets, and the related parameter sea level are discussed. In all cases, scientists attempt to monitor both the areal extent and mass of these snow and ice bodies. Areal extent is easier to determine than mass. Various forms of remote sensing, from both aircraft and satellite, allow us to look down on surfaces at varying spatial scales and over time to determine if the snow or ice covered area is expanding or contracting. Long-term monitoring includes looking at the areal extent of snow cover and sea ice, as well as changes in area and mass of mountain glaciers.